The present invention relates to thermal overloads and, more specifically, to thermal overload protection for locomotive traction alternators.
It is well known in the art to provide overload protection for rotating electrical machinery. Existing methodology employed relies on the use of stator winding temperature sensors to provide a signal for either alarm, load reduction, or "trip" of the alternator, if temperatures beyond a given limit are encountered. This would usually be the result of some type of electrical overload. The protective action function would be carried out by either reducing the load manually or tripping the alternator circuit breaker.
Overload protection for rotating electrical machinery is intended to provide a means of preventing excessive overheating of the electrical winding insulation system. The protection used has been similar for most types of rotating electrical machines and the devices employed typically fall into one of several categories. For example, the device employed may be a time-delay overcurrent device or relay, a thermal relay or device operated by machine electrical current, a temperature relay or device operated by an embedded winding temperature sensor or thermostat, or a combination of these devices.
The protection option chosen is usually to provide an alarm to an operator, to disconnect or trip the alternator from its load, or to permit short overload excursions compatible with the inverse-time overload limit curve of the machine. The use of embedded winding sensors is currently the preferred method for thermal protection. Most large machines usually employ Resistance Temperature Detectors (RTD's) embedded in the stator windings for the purpose of directly sensing stator winding temperatures.
Unfortunately, locomotive traction alternators have not typically been protected against damaging thermal overloads due to the philosophy that permissible traction motor short-time rating limits will prevent alternator temperatures from reaching excessive levels. It is seen then that there exists a need for a thermal protection control which provides a smooth and gradual reduction in the alternator electrical load.